504. Base 7

solution504.py

# https://leetcode.com/problems/base-7/description/

class Solution(object):
    BASE_EXP = [1, 7, 49, 343, 2401, 16807, 117649, 823543, 5764801, 40353607]
    def convertToBase7(self, num):
        """
        :type num: int
        :rtype: str
        """
        def add_x_to_digits(x, digit_list):
            list_ptr = 0
            lsd = digit_list[list_ptr]
            
            carry = 1 if (lsd + x) > 6 else 0
            
            if carry == 0:
                digit_list[list_ptr] += x
                return
                
            while carry == 1:
                if list_ptr >= len(digit_list):
                    digit_list.append(0)
                digit = digit_list[list_ptr]
                digit += x
                if digit > 6:
                    digit -= 7
                    carry = 1
                else:
                    carry = 0
                x = carry
                digit_list[list_ptr] = digit
                list_ptr += 1
                
        neg = True if num < 0 else False
        num = abs(num)        
        digits = [0] * 10
        i = len(digits) - 1
        
        rem = 0
        
        while i > 0:
            digits[i] = num//self.BASE_EXP[i]
            rem = num % self.BASE_EXP[i]
            if digits[i] > 0:
                num = rem
            i -= 1
        
        digits[i] = rem
        
        digits = digits[::-1]
        
        for i in range(len(digits)):
            digits[i] = str(digits[i])
        
        ret_str = str(int(''.join(digits)))
        
        if neg:
            ret_str = "-" + ret_str
            
        return ret_str